This invention relates to an electron gun assembly for emitting three electron beams.
A cathode ray tube, such as an electron gun assembly of a color picture tube, generates three electron beams, which are directed onto a target screen coated with a phosphor layer, to cause the screen to emit light rays. To improve the image sharpness of the color picture tube, it is necessary to reduce the diameters of beam spots projected onto the screen, i.e., to improve the focusing characteristics of the electron guns, and to converge the three beams at a predetermined point near the screen.
An electron gun assembly essentially consists of an electron beam forming region for generating electron beams, and a main lens system for accelerating and focusing the beams onto target screen. Generally, the lens system is provided with means for converging the three electron beams at a predetermined point near the screen. Most of the lens system are electrostatic lenses, which are formed on an electron beam path by coaxially disposing a plurality of electrodes each having apertures, and applying predetermined potentials to the respective electrodes. Several types of lenses are employed, in accordance with the difference in the shapes of the electrodes and the voltages applied. There is a method for improving lens performance, by forming a large-aperture lens, through increasing the diameter of the aperture of the electrode. There is also a method of forming a long-focal length electron lens, by increasing the distance between electrodes, to provide a smooth change in potential. Since an electron gun for a cathode ray tube is sealed in a neck tube having a relatively small diameter, the size of the electrodes is therefore limited. Consequently, any increase in the size of the aperture of the electrode is also limited. As an interval Sg between electron guns is increased as the diameters of the apertures of the electrodes are increased, the electrical power, required for deflecting an electron beam, increases. Such an increase in power consumption is undesirable.
Since the electric field of the adjacent electrode and an undesired electric field from a neck wall affect the trajectory of the electron beams when the distance between the electrodes is merely lengthened, the distance cannot be increased without limit.
A method of increasing a distance between electrodes without affecting the electron beams from the abovementioned undesired electric fields is disclosed in U.S. Pat. No. 3,932,786, granted to Campbell. Campbell discloses an electron gun assembly, for a color image tube, having bipotential lenses in a main lens system. More particularly, a number of metal plates are located, as intermediate electrodes, between the first and second focusing electrodes in the electron gun assembly disclosed in this U.S. Patent, the first and second focusing electrodes being connected via a ceramic film resistor, and the intermediate electrodes being coupled by resistors. Therefore, when an anode electrode voltage is applied from the anode of the second focusing electrode and a focusing voltage is applied from an external power supply to the first focusing electrode, a smooth potential gradient is formed from the first focusing electrode through the intermediate electrodes to the second focusing electrode, to form a long-focal length electron lens in which the distance between the first focusing electrode and the second focusing electrode has been increased.
However, this electron gun structure has the defects in that, if the focusing voltage applied to the first focusing electrode is adjusted, the potentials of the intermediate electrode are changed, and, as a result, misconvergence of the three electron beams (called "static misconvergence") occurs.
A method of converging three electron beams in a conventional electron gun assembly will now be described with reference to FIGS. 1 and 2.
FIG. 1 shows an example of a bipotential type electron gun assembly. Main lenses 63R, 63G, 63B are formed between first focusing electrodes 61 and second focusing electrodes 62. In this electron gun assembly, the opposing end faces of the electrodes corresponding to the side guns are formed obliquely with respect to the axes of the electron guns. Therefore, the electric fields for forming lenses 63R and 63B are formed asymmetrically with respect to the axes of the electron guns, and electron beams 65 and 66, passing through the side guns, are resultantly directed toward the center gun, so that three electron beams are converged at a predetermined point near the target screen. In the electron gun assembly shown in FIG. 2, central axis 67 of the aperture of second focusing electrode 62, corresponding to the side gun, offsets from central axis 68 of the aperture of first focusing electrode 61, in a direction away from central axis 69 of the center gun. Thus, the electric fields for forming the main lenses are formed asymmetrically with respect to the axes of the electron guns, i.e. the axes of the apertures of the electrodes, and the electron beams passing through the side guns are resultantly deflected toward the center gun, in the same manner as the case of FIG. 2, so that three electron beams are converged at a predetermined point near the target screen.
In U.S. Pat. No. 3,932,786, the method of converging the electron beams is not disclosed, but in order to display a clear image on the target screen, one of the above-mentioned methods must be employed in the main lens system. Therefore, it is assumed that in the electron gun structure disclosed in this U.S. Patent, as is shown in FIG. 2, the aperture axes of the second focusing electrodes of the side guns are offset from those of the intermediate electrodes thereof. An electron gun structure thus conceived has drawbacks in that, if the voltage applied to the first focusing electrodes is adjusted, the voltage of the intermediate electrodes is changed, so that the accuracy of convergence of the electron beam, i.e., static convergence, is degraded. In other words, this electron gun structure has the drawback in that the accuracy of convergence of the electron beams is affected by the adjusting of the focusing voltages, and, as a result, fine adjustment of the focusing and converging of the electron beams becomes difficult.
A method of correcting a misconvergence of the electron beam (static misconvergence), caused by the adjusting of the focusing voltages, is disclosed in U.S. Pat. No. 4,334,169, granted to Takenaka et al. U.S. Pat. No. 4,334,169 discloses a quadrapotential type electron gun assembled as shown in FIG. 3. In this assembly, an auxiliary lens is formed by third grid 71, fourth grid 72, and fifth grid 73, and a main lens is also formed, by fifth grid 73 and sixth grid 74. Furthermore, mounted the axes of the apertures corresponding to the side guns between fourth and fifth grids 72 and 73 and between fifth and sixth grids 73 and 74 offset from the axis between the third grid 71 and fourth grid 72 to converge the electron beams. A focusing voltage of about 7 kV is applied to third and fifth grids 71 and 73, a voltage of about 600 V is applied to fourth grid 72, and a voltage of about 25 kV is also applied to sixth grid 74.
If the focusing voltage is, for example, raised in the above-mentioned structure, the lens force of first auxiliary lens 80 is increased, and the deflecting force of the side beam is accordingly strengthened, while the lens force of second main lens 81 is weakened, and the deflecting force of the side beams is correspondingly weakened, so that a variation in static convergence is resultantly prevented.
When the focusing voltage decreases, a reverse phenomenon to the above occurs, so that a variation in static convergence is also prevented.
As has been described above, this electron gun assembly has a characteristic in that the static convergence is always maintained. However, it is necessary to offset the axes of the apertures of the side guns in the two regions. Therefore, the electrode structure forming the gun assembly is complicated, and the assembling and manufacturing of the assembly are thus also complicated.
Further, the side beams are deflected in two stages, and the electron beams pass through two lenses having electrical distortions, so that the distortions of the side beams consequently tend to increase. Therefore, the electron beam focusing performance of the center and side beams are not uniform, thereby sacrificing the focusing performance of the electron gun.
As has been described above, in the conventional electron gun assembly, the convergence of the electron beam is affected by the adjustment of the focusing voltage, and, as a result, fine adjustment of the focusing and converging of the electron beams becomes difficult. Furthermore, the electron gun assembly for eliminating the above drawbacks has another drawback, in that it is complicated in its structure.